1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic method such as a printer, a copier, a facsimile machine, and a multi-function apparatus having one or more capabilities of the above devices, and in particular to a secondary transfer device included in such an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus employing the electrophotographic method and having an intermediate transfer member, in general, an electrophotographic image formed on an image carrier such as a photoreceptor is developed a toner image by a developing device; the toner image is once transferred from the image carrier to an intermediate transfer member such as an intermediate transfer belt; thereafter, the toner image on the intermediate transfer member is secondarily transferred to a recording medium such as a transfer sheet at a secondary transfer portion or nip formed of the intermediate transfer member and a secondary transfer roller and an opposed roller that sandwich the intermediate transfer member; and the toner image formed on the recording medium is fixed thereon by a fixing member with pressure and heat.
There is an acute demand for optimal toner image transfer to various recording media available in the market while maintaining good image quality across all media. However, currently commercially available recording media for use in the on-demand printing field typically have surfaces with various asperities therein, and such rough surfaces tend to adversely affect image quality.
One way to form an optimal image on such recording media involves improving the transferability of the toner image to the recording media by using greater pressure to press together the secondary transfer roller and the opposed roller forming the secondary transfer nip with the intermediate transfer member sandwiched in between.
However, increasing the nip pressure in the secondary transfer nip has a disadvantage in that, when a relatively thick sheet is passed through the secondary transfer nip set at a high pressure, fluctuation in the torque load on the intermediate transfer member greatly increases, which in turn causes the rotation speed of the intermediate transfer member to fluctuate. More specifically, the fluctuation in speed occurs in such a manner that, when a thick recording medium enters between the secondary transfer roller and the opposed roller sandwiching the intermediate transfer member, the load on the opposed roller greatly increases and the brakes are applied to the rotation speed of the intermediate transfer member. As a result, tension is given to the secondary transfer nip upstream that causes the intermediate transfer member to go slack. At the same time, the intermediate transfer member is stretched taut downstream, thus changing the rotation speed of the intermediate transfer member.
When image formation is to be performed on multiple sheets of recording media, while a previous toner image is being transferred to the recording medium at the secondary transfer nip, a subsequent image is primarily transferred to the intermediate transfer member at the primary transfer portion. In this case, when the speed of the intermediate transfer member changes due to the passage of the recording medium into the secondary transfer nip, the next toner image being primary transferred to the intermediate transfer member may acquire lateral bands as the recording medium enters the secondary transfer nip and the toner image is secondarily being transferred onto the recording medium. This phenomenon is called shock jitter. Accordingly, that the secondary transfer nip portion is applied with a high pressure to exert an optimal transferability to recording media with surface asperities may result in disadvantage that an optimal transferred image cannot be obtained in repeatedly printing thick sheets.
JP-2008-96557-A discloses a structure to reduce the torque load change of the intermediate transfer member in the secondary transfer nip. The same includes a first rotary member as an opposed roller, a transfer roller as a second rotary member to which biasing force to bias the opposed roller is given from a biasing means, and discloses a structure in which, when a recording medium is supplied between the first and second rollers, a rotary shaft of the transfer roller escapes in a direction different from the biasing direction of the biasing means so that a pushing-up force of the transfer roller which vertically shifts from the conveyance direction of the recording medium is reduced depending on the thickness of the sheet conveyed and the torque load change of the intermediate transfer member sandwiched by the transfer roller and the opposed roller can be reduced.
Even with the above-described configuration, however, it is difficult to simultaneously achieve both improvement in image transferability to recording media with surface asperities and reduction of shock jitter when the thick sheet enters the secondary transfer nip.